Current practice in the use of MEGA-PRESS spectroscopy for the detection of GABA

Abstract

There is increasing interest in the use of edited proton magnetic resonance spectroscopy for the detection of GABA in the human brain. At a recent meeting held at Cardiff University, a number of spectroscopy groups met to discuss the acquisition, analysis and interpretation of GABA-edited MR spectra. This paper aims to set out the issues discussed at this meeting, reporting areas of consensus around parameters and procedures in the field and highlighting those areas where differences remain. It is hoped that this paper can fulfill two needs, providing a summary of the current 'state-of-the-art' in the field of GABA-edited MRS at 3T using MEGA-PRESS and a basic guide to help researchers new to the field to avoid some of the pitfalls inherent in the acquisition and processing of edited MRS for GABA.

Keywords: Edited MRS; GABA; MEGA-PRESS; MRS analysis.

Fig. 1

Schematic diagram of MEGA-PRESS editing for GABA. (a) Editing pulses applied at 1.9 ppm modulate the shape of the GABA signals at 3 ppm (b). Subtracting scans acquired without these pulses (labeled OFF) from scans acquired with the editing pulses (ON) removes overlying creatine signals from the edited spectrum, revealing the GABA signal in the difference spectrum (labeled DIFF). (b) shows the effect of editing pulses on signals at 3 ppm only.

Fig. 2

Comparison of GABA-edited spectra across three vendors. The Siemens data is acquired with a vendor-distributed sequence, whereas the GE and Philips sequences are customer implementations by and available from RAEE. (a) Phantom data (acquired in a 10 mM GABA solution in phosphate-buffered saline) show similar edited signal in each case. Note that the commonly anticipated ‘pseudo-doublet’ is not observed in any implementation, and that implementations differ significantly in the extent to which the ‘center peak’ is edited. (b) In-vivo edited spectra. Typical parameters are: TE 68 ms; TR 2 s; 3×3×3 cm³ voxel; acquisition time 10 min; editing pulses applied at 1.9 ppm (ON) and 7.46 ppm (OFF).

Fig. 3

Representative PRESS (a) and MEGA-PRESS (b) pulse sequences, showing the addition of the editing pulses symmetrically around the second 180° pulse.

Fig. 4

The effect of fit model on the estimation of GABA: Analysis was performed on 144 in-vivo datasets using both single Gaussian and double Gaussian functions to fit the GABA peak. The high correlation between the resulting integral values demonstrates that, for in-vivo data, fitting a single Gaussian is equivalent to using a more complex model.